Correlation assessment between actual pollution performance of insulator strings in DC and theoretical models

The performance of overhead line insulation in DC differs from AC mostly because of ionization of the airborne particles resulting from a unidirectional electric field. As a consequence, contamination levels in DC is typically more severe than for AC in the same environment. CIGRE and IEC have published technical guides and mathematical models to handle this situation at the stage of designing the insulation of a DC line. This paper gives a comparison between the insulation levels defined by these new models, actual field observations and laboratory test results. The difference pointed out show the necessity to engage into a new evaluation of the models which otherwise could lead to string lengths largely over dimensioned given their pessimistic approach.